Paper is made as a continuous web on a papermaking machine. The machine has a wet end where papermaking stock, composed of over 99 percent water is fed onto a moving wire screen known as a Fourdrinier. In order to produce a more one-sided web, two forming fabrics are often used in what is known as a "Twin Wire Former" where water is drained from both sides to form the web. After the water drains through the screen or screens it leaves a thin sheet of fibers forming the web of paper. The web as formed still contains over 80 percent water. From the forming screen or wire the web is moved through a pressing section where water is pressed from the web. Upon leaving the pressing section, the web of paper is still composed of approximately 60-65 percent water. The pressed web is then dried on a series of steam heated drums before being wound onto a reel at the dry end of the papermaking machine.
In forming a paper web it is important, particularly in the lighter weight grades of paper used for printing newspapers and magazines, that both sides of the sheet of paper formed be essentially identical. Paper which has similar attributes on both surfaces can readily be printed on both sides with a uniform result. Where both sides of a paper sheet are essentially identical the paper is referred to as one-sided. Two-sided paper, where the properties of each side differ significantly, is undesirable and can result from more water being removed from one side of the web than the other in the pressing section. Pressing sections are therefore generally designed to maintain one-sidedness in the web of paper being formed.
Drying paper requires more energy than pressing the water from the paper web. On high speed modern papermaking machines where the web may move through the machine at speeds in excess of 6,000 feet per minute, the length of the dryer section needed can become excessively long in order to dry the rapidly moving web. This has led to the use of high temperature press rolls. High temperature press rolls of either the conventional or Extended Nip.RTM. press (ENP) manufactured by Beloit Corporation, of Beloit, Wis., can increase the dryness of the paper, significantly reducing the amount of drying required. However, a portion of a conventional dryer section is still required.
A recently developed technique for increasing the rate of drying of a paper web is described in U.S. Pat. No. 5,127,168 to Pikulik. The described technique involves pressing a paper web into intimate contact with a dryer roll which increases the rate of heat transfer from the dryer drum to the web. The adhesion of the web to the dryer rolls allows the use of aircaps on the dryer rolls to increase the rate of drying.
Increasing the drying rate of a paper web being formed is an important development. Improvements in papermaking technology have in the past resulted in wider machines running at higher speeds. Accompanying these improvements the papermaking machines themselves have increased in size. The future appears to be in papermaking machines which operate at much higher speed and employ high intensity pressing and drying sections which significantly reduce the overall size of the papermaking machine. At the same time that the papermaking machine is getting shorter the quality of the fibers used to manufacture paper is decreasing because of the increased cost of virgin fiber and the demand for greater use of recycled fiber.
Therefore a dryer section or pressing section and dryer section combination is needed which increases paper strength and reduces dryer section length.